Details of the Structure of DNA Were Worked Out by a Number

Details of the structure of DNA were worked out by a number of scientists. At first, DNA was interpreted as being too regular in structure to carry genetic information but, by the 1940s, DNA from different organisms was shown to vary in its base composition.
-Levene made which contribution of our understanding of DNA structure?

A) Determined that the nucleus contains DNA.
B) Determined that DNA contains four nitrogenous bases.
C) Determined that DNA consists of nucleotides.
D) Determined that the nucleotide bases of DNA are present in regular ratios.